1. Technical Field
The present invention generally relates to battery charging methods and apparatus, and particularly relates to a battery charging method and apparatus having certain charging properties that depend on the charging source while having other charging properties that depend on battery capacity regardless of the charging source.
2. Background
Rechargeable batteries appear in a growing range of electronic devices. The use of rechargeable batteries is particularly common in portable electronics, such as cell phones, Portable Digital Assistants (PDAs), pocket and notebook computers, Global Positioning System (GPS) receivers, etc. No one rechargeable battery type finds universal usage across this diverse range of devices, as each battery type offers its own set of tradeoffs regarding performance, size and cost.
For example, types of rechargeable batteries include, but are not limited to, lead-acid cells, nickel-cadmium cells, nickel-metal hydride cells, sodium-sulfur cells, nickel-sodium cells, lithium-ion cells, lithium polymer, manganese-titanium (lithium) cells, nickel-zinc cells, and iron-nickel cells. Each of these battery chemistries offers its own mix of advantages and disadvantages regarding size, energy density (volumetric or gravimetric), cost, cell voltage, cell resistance, safety, toxicity, etc.
Despite such differences, some charging algorithms find broad applicability across a wide range of battery chemistries. As an example, the constant-current/constant-voltage (CC/CV) charging algorithm is adaptable to many different types of battery chemistries and, therefore, finds wide usage in a variety of battery charging products. With the CC/CV charging algorithm, a discharged battery is charged at a constant current until its cell voltage rises to a defined threshold voltage, e.g., the battery's “float voltage,” at which point the charging control is switched to constant-voltage to charge the remaining capacity of the battery without exceeding the voltage limit of the battery.
In order to maintain constant current and constant voltage, the CC/CV charging algorithm initially relies on current feedback control and then switches over to voltage-feedback control, once the battery-under-charge reaches its float voltage. The charging algorithm may, however, continue to monitor the current during constant-voltage control in order to detect when the battery has completed charging. Detecting when the battery has completed charging and correspondingly terminating the charging current is important for avoiding damage to the battery and preserving battery life.
Because the amount of charging current that a battery will accept decreases as the battery approaches full charge, the algorithm may terminate charging when the monitored charging current falls to a defined termination current. Ideally, the termination current should be set to yield maximum battery charging while avoiding potentially dangerous excess charging of the battery. This termination current detected during the CV phase of recharge and the charging current permitted during the CC phase of recharge may both depend on the battery's capacity.
To illustrate, the charging current permitted during the CC phase of recharge should be as high as possible within recommended limits because higher charging currents equate to lower recharge times. Battery manufacturers often rate battery capacity in terms of a given battery's “C rating,” which is a scaling unit for the battery's charge and discharge currents. Charging or discharging the battery at rates beyond the “C” rating exceeds the safe rating of the battery. For example, many manufacturers' recommendations specify charging currents not exceeding 1C for safety reasons. Thus, charging current during the CC phase of recharging may be set in proportion to the battery's capacity by setting the charging current at the manufacturer's recommended C rate limit.
Similarly, the termination current detected during the CV phase of recharge may also be set according to the recommended C rate limit as the current that a battery draws when fully charged likewise depends on the battery's capacity. When both the charging current and the termination current depend on the battery's capacity, the termination current is necessarily smaller than, although proportional to, the charging current. As a matter of designing a battery charging system, then, the charging current may be set in proportion to the battery's capacity and the termination current set as a fixed fraction of the charging current. The termination current may, for example, be set to detect a fully charged battery when the charging current falls to 10 percent of the charging current permitted during the CC phase of recharge.
One of the many challenges faced by designers of battery charging systems arises, however, when the battery charging system permits charging a battery-powered device from various types of sources, which may or may not be capable of charging the battery according to the appropriate capacity-based charging strategy. For example, a given charging source may not be able to charge at the battery's recommended C rate. Instead, characteristics of the source may constrain the charging current permitted during the CC phase of recharge. More particularly, many battery-powered devices interface to Personal Computers (PCs) and the like via Universal Serial Bus (USB) connections having a voltage bus (VBUS). Portable music players, such as those based on the popular MP3 digital audio format, are just one example of such devices.
Regardless, the USB standard defines a low-power device as one that draws no more than 100 mA of current, and defines a high-power device as one that draws up to 500 mA. Devices that would like to draw current up to the high-power 500 mA limit must first request, and be granted, permission to do so from the USB port host. Until granted such permission, the requesting device must draw no more than 100 mA. Thus, to comply with the USB specification, a device wishing to charge its battery at the high power current limit must also support the low power current limit, and be able to switch between the two limits.
Thus, some USB ports may not allow battery charging currents above the low-power limit of 100 mA. Moreover, some adapter charging sources, especially those not dedicated to the particular battery-powered device being charged, may be incapable of supplying current at the battery's C rate. Thus, the battery's capacity may permit a greater charging current than a charging source is capable of providing.
Instances where the charging source cannot provide the appropriate CC phase charging current present a number of problems. For example, it is known to set the termination current as a fraction of the CC phase charging current, reflecting the assumption that the CC phase charging current is appropriate in magnitude for the capacity of the battery being charged. Thus, to the extent that the CC phase charging current does not have the appropriate magnitude given the battery's capacity, the termination current necessarily will have the wrong magnitude in relation to the battery's capacity. In a similar fashion, the pre-charge current, used to prepare a deeply discharged battery cell for the CC phase charging current, also depends on the battery capacity. Setting the pre-charge current as a fraction of the CC phase charge current thus is appropriate only to the extent that the charge current itself is set according to battery capacity, rather than according to one or more source constraints.